To overcome difficulties encountered as a result of deformation of portions of actuators it has been suggested that pistons or glands of the actuator be constructed to provide frangible portions which upon striking the deformation will fracture thereby allowing the actuator to continue to operate. Such frangibility is provided through utilization of a plurality of frangible segments formed by a plurality of radially extending slots in opposed flanges along with a score line at the base of the slots. Such structure is disclosed in U.S. Pat. No. 3,884,127.
Although such structure operates satisfactorily in most instances, it has been discovered that in those applications where a relatively small clearance exists between the internal cylinder wall and the piston rod outer diameter previously unforeseen difficulties are encountered.
One such difficulty is the fact that the segments are relatively short thereby providing a small moment arm and thus a larger amount of force is required to cause a fracture of the segment. Under some circumstances, the amount of force required is so great that the actuator including any additional stages in a redundant system is incapable of generating sufficient force to effectuate a fracture of the segment.
An additional difficulty which has been encountered in the small clearance actuators is that the typical deformation of a cylinder wall, as by a projectile penetrating the same, causes the deformed portion of the wall to be immediately adjacent the external surface of the piston rod of the actuator. When utilizing the prior art structures, assuming the segments have, in fact, fractured and broken off, there is typically a small rim or protrusion extending upwardly from the surface of the piston rod at the point of the fracture. This protrusion contacts the deformed portion of the cylinder wall and prevents further movement of the piston relative to the cylinder.
A further problem which has been encountered with the prior art structures in small clearance devices is that the segments when they fracture and break off often wedge between the piston rod and the internal wall of the cylinder. This wedging phenomina is caused to occur because a rim at the periphery of the segments is provided which causes the widest portion of the segments to be at the outer periphery thereof. When a force causes the segments to fracture they are then caused to rotate inwardly (toward the seal groove) and the distance between the outer corner of the segment and the inner corner at the point of fracture is much greater than the clearance in existance between the cylinder wall and the piston rod thereby causing the fractured segment to wedge in place thus affecting deleteriously the operation of the apparatus.